The present invention relates to a pickup device for reproducing information recorded on an optical disk used as an optical recording medium for recording information such as video signals. More particularly, the invention relates to pickup device having an improved signal reproduction characteristic.
The optical disk based on the laser technology has been known as a large capacity recording medium for storing large lo amount of information, such as digital data signals and analog data signals, e.g., video data signals. The reproduction system for reproducing information from an optical disk which has been used prevalently is of the reflection type in which a laser beam reflected on the recording surface of the optical disk is used for data reproduction. The reason why this type of data reproduction system is widely used is that it is easy to manufacture and simple in construction, and allows reduction of the size of the drive system.
The optical disk of the reflection type is classified into an optical disk of the reproduction only type, an optical disk of the additive recording type, and an optical disk of the rewritable type. A series of pits, representative of data signals to be recorded, are spirally formed on the recording surface of the optical disk from the inner side of the disk toward the outer side. One turn of the series of pits as unit data forms one recording track. To reproduce the data of the pits, the positioning of the pickup device is carried out for each track. The term "pit" generally refers to a hole or cavity. In the description to follow, for ease of explanation, the term "pit" also means data recording modes in which data is recorded without any change in the shape of the disk, such as magnetic-optical recording mode and phase change recording mode.
FIG. 7 is an explanatory diagram showing the scheme of a prior art pickup device. In the figure, reference numeral 101 designates a semiconductor laser device having a traverse single mode. The laser device serves as a point light source having a light emitting point of approximately 0.1 .mu.m in diameter. Numeral 102 represents a half mirror for separating a light beam projected onto an optical disk 100 and the reflected light beam from the optical disk 100. Numeral 103 designates an objective lens, and numeral 104 indicates a photo detector for detecting the reflected light beam. In operation, the light radiating from the laser device 101 is reflected by the half mirror 102, and is focused on the recording surface of the optical disk 100 through the objective lens 103. In this case, the wave length .lambda. of the light source and the numerical aperture NA of the objective lens 103 are selected to be .lambda./NA&gt;0.1 .mu.m. Accordingly, the so-called "diffraction limit" acts to limit the size of the light spot on the disk to .lambda./NA.
As already described, data signals are recorded in the optical disk 100 in the form of a series of pits whose nature is optically changed. An optical disk of the type in which portions of different reflectivities is formed therein will be described. In this type of the optical disk, data signals are recorded into the disk by forming portions of different reflectivities. To form the portions of different reflectivities, coloring matter previously mixed into the record film of the disk is exposed to light, or the record film is placed to a crystalline state or an amorphous state. Intensities of the laser beams reflected from the optical disk 100 are modulated according to the different reflectivities on the optical disk 100, and are concentrated again by the objective lens 103. Part of these laser beams pass through the half mirror 102. By the astigmatism faculty of the half mirror 102, the laser beams are spread out with respect to the optical axis to have different focal positions as viewed in the vertical and horizontal directions. The photo detector 104 is located at the mid-point between the two different focal positions.
The photo detector 104 may be constructed with a quartered photo diode array, as shown in FIGS. 8A to 8C. The photo detector 104 is positioned in that at a far focal position, the light spot is shaped in cross section as shown in FIG. 8A; at a near focal position, it is shaped as shown in FIG. 8C; and at the focal position, it is shaped as shown in FIG. 8B. A difference of light intensity between the diagonal line components of light on the photo detector 104 is calculated to obtain the quantity of defocus. The focal position is controlled on the basis of the defocus quantity. In this case, the light spot projected on the optical disk 100 is not focused on the detect surface of the photo detector 104, but it is detected in a state that it is displaced by the half of the focal position difference, viz., in the so-called far field pattern. Meanwhile, in Examined Japanese Patent Publication (Kokoku) Sho-52-50131 (the first prior art document), there is disclosed a technique in which the signal is detected at the focal position. The technique is able to minimize the light spot movement on the photo detector.
A high density read method is proposed in Unexamined Japanese Patent Publication (Kokai) Sho-57-58248 (the second prior art document). In the proposal, a plurality of light sources are provided. Three recording tracks are radiated with light beams from those light sources. Far field patterns of the light sources are detected by the photo detector. The output signals of the photo detector are subtracted from one another according to the leakage rates previously measured, thereby to reduce the quantity of signal leakage such as crosstalk.
In the pickup device of the first prior art, in the case where a record density on the optical disk is increased by reducing the track-to-track distance without changing the wavelength of light from the light sources and the numerical aperture, the light spot must be influenced by changes of the reflectivities of the tracks adjacent thereto. Accordingly, when the signal is read out, much leakage from the adjacent tracks is mixed into the readout signal. This leads to degradation of S/N ratio performance.
In the second prior art document, it is necessary to form a light spot by the diffraction limit. When the track-to-track space is made to be narrower than the spot diameter, the influence by the adjacent tracks will be created. Thus, it can not be expected large effect.